Leucine degradation in sheep

Abstract

1. In vitro leucine catabolism in adipose tissue, skeletal muscle, kidney and liver homogenates was studied in sheep.2. In Expt 1, Suffolk × Targhee ram lambs were slaughtered at 1, 28, 56, 84, 112, 140, 168, 196, 224 and 365 d of age. In Expt 2, 5-month-old crossbred wethers were fed on 80, 120 or 180 g crude protein (nitrogen × 6·25) /kg diets for 4 weeks or fed on 120 g crude protein/kg for 4 weeks and then fasted for 48 or 96 h before slaughter. Leucine catabolism was assayed in tissue homogenates for Expts 1 and 2.3. Leucine deamination (per unit protein) was highest in skeletal muscle at day 1 and then declined; liver exhibited an activity pattern akin to muscle while kidney activity tended to rise over the duration of the study. Adipose tissue in vitro leucine deamination was higher at all ages studied and 9- to 50-fold higher than all other tissues at 365 d. Leucine decarboxylation (per unit protein) was highest at day 1 in muscle and declined to low levels (P < 0·01) after 28 d; liver and kidney decarboxylation activities were higher than muscle at all ages with kidney showing the highest activity. Whilst adipose had high initial activity it declined significantly (P < 0·01) by day 28 and remained low.4. Dietary protein intake had no effect on leucine deamination in any tissue. Leucine decarboxylation tended to increase with protein intake for all tissues except kidney. Length of fast (96 h) resulted in a variable decline in leucine deamination; leucine decarboxylation was significantly lower in kidney, liver and adipose tissue after a 96 h fast.5. When these in vitro enzyme activity results are related to questions concerning the role of skeletal muscle in leucine catabolism in sheep, the present findings indicate that in grown sheep, skeletal muscle has a small to moderate role in total body leucine deamination but plays a very minor role in leucine decarboxylation. Adipose tissue appears to be a major site of leucine deamination in grown sheep. These results are not in agreement with the idea that during fasting leucine becomes an important energy substrate and is oxidized in skeletal muscle as has been shown in rodents.

Changes in tissue abundance and activity of enzymes related to branched-chain amino acid catabolism in dairy cows during early lactation

“…The mRNA abundance of BCATm was greatest in both scAT and vAT compared with all other tissues, which indicates that, on a transcriptional level, AT might be an important contributor in the initiation of BCAA catabolism in dairy cattle. This notion is in line with a study by Bergen et al (1988), who showed that Leu deamination activity as well as the predicted deamination capacity based on body and tissue weights was highest in scAT compared with muscle, liver, and kidney of mature sheep. More recently, Green et al (2016) demonstrated the importance of BCKA oxidation in adipogenic differentiation and that inhibition of BCAA catabolism compromised adipogenesis.…”

“…Even if full hepatic BCKA oxidation may be relatively low, high BCKDH activity would ensure the removal of excess BCKA, which are toxic at high blood concentrations. In general, it has been acknowledged that the initial step in BCAA catabolism, the reversible deamination via BCAT, may occur extrahepatically [e.g., in muscle (Bequette et al, 2002), AT (Bergen et al, 1988), or MG (Bequette et al, 1996a)]. Depending on the tissue, this is followed by either the export of the resulting BCKA into the blood stream or direct oxidative decarboxylation of BCKA to acyl-CoA derivatives via BCKDH and, hence, not adding to the plasma flux .…”

Effect of food intake on hind-limb and whole-body protein metabolism in young growing sheep: chronic studies based on arterio-venous techniques

“…Leucirze oxidation. The liver is the major site of catabolism for most amino acids but substantial oxidative capacity for the branched-chain amino acids (leucine, isoleucine and valine) resides also in ovine skeletal muscle and adipose tissue (Goodwin et al 1987;Bergen et al 1988), which contain both the transaminase and the branched-chain 0x0-acid dehydrogenase. The activity of the dehydrogenase is tightly regulated by the phosphorylation status; the phosphorylated form is inactive and the dephosphorylated is active (Randle et a/.…”

Purification and cloning of the mitochondrial branched‐chain amino acid aminotransferase from sheep placenta

“…Another difference between ruminants and monogastrics is in the developmental expression of BCAT in ruminants. Sheep BCAT activity declines throughout development, from fetus to ruminant, especially in skeletal muscle [11,13,14,29]. Such a decrease in BCAT expression during development has not been described in any other species [31].…”

“…We have shown previously that BCAT activity is very low in preruminant lamb tissues [11]. Studies carried out at other developmental stages (fetus, growing lambs, adults and pregnant ewes) confirm that BCAT activity is lower in sheep than in rat tissues, and indicate that BCAT activity decreases during development [13,14]. As the BCAT isoenzymes have not yet been purified from sheep, the biochemical basis for the differences between ruminants and monogastrics is not known.…”